PARTITIONING OF F, CL, AND H2O BETWEEN APATITE AND A SYNTHETIC BASALTIC LIQUID: IMPLICATIONS FOR QUANTIFYING WATER CONTENTS OF LUNAR MAGMAS FROM APATITE

The mineral apatite is present in a wide array of planetary materials, and interest is growing in using apatite as a tool to estimate magmatic volatile abundances. In order to use the volatile concentrations of apatite to precisely determine the abundances of volatiles in coexisting silicate melt or fluids, thermodynamic models for the apatite solid solution and for the apatite components in multi-component silicate melts and fluids are required. Although some thermodynamic models for apatite have been developed, they are incomplete. Furthermore, no mixing model is available for the apatite components in silicate melts or fluids (particularly for the halogen components). In lieu of overcoming these issues, we conducted apatite-melt partitioning experiments from 1.0-4.0 GPa and 950-1750 °C on a synthetic iron-rich basalt composition (similar in composition to many lunar and martian basalts) to better understand the partitioning behavior of F, Cl, and OH between apatite and silicate melt.

The apatite-melt partition coefficients for H₂O range from about 0.025-0.25, for F they range from about 0.15-40, and for Cl they range from 0.3-5. Consequently, mineral-melt D values are not appropriate to use for quantifying volatile abundances in magmatic liquids from apatite composition alone. This is to be expected because F, Cl, and OH are essential structural constituents in apatite. However, we observed that exchange coefficients are not as variable and change systematically as a function of temperature and melt composition. In fact, the exchange coefficients indicate that F is preferred in the apatite structure by a factor of 100 compared to OH and by a factor of 20 compared to Cl. Cl is preferred by a factor of 20 over OH. Based on these results, the estimates of water contents in lunar magmas, which primarily used and apatite-melt D value of 0.25, are appropriately conservative for estimating lower limits on magmatic water abundances in late-stage lunar liquids.